Macromolecules 1994,27, 832-837
832
Photodegradation of Polyimides. 6. Effect of Donor-Acceptor Groups on the Photooxidative Stability of Polyimides and Model Compounds David Creed3 Charles E. Hoyle,'*t**Petharnan SubramanianJ Rajamani Nagarajan,? Chandra PandeyJ Edgardo T. Anzures) Kevin M.Cane3 and Patrick E.Cassidys Department of Polymer Science, University of Southern Mississippi, Hattiesburg, Mississippi 39406-0076, Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406-5043, and Polymer Research Group, Department of Chemistry, Southwest Texas State University, Sun Marcos, Texas 78666-4616 Received May 27, 1993; Revised Manuscript Received September 29, 199P ABSTRACT The photophysics of polyimides and several N-arylphthalimide model compounds has been investigated as part of an effort to understand the factorsthat controlthe oxidative photodegradationof these materials. Substituents that increase the donor-acceptor (D-A) character of both polymers and models cause an enhancement of the red-shifted electronic absorption and a shift to lower energy of the broad, weak fluorescence of these materials. Increasing solvent polarity causes a similar effect on the absorption and fluorescence spectra of the model compounds. Fluorescence quantum yields decrease with increasing D-A character in the series of model compounds and with increasing solvent polarity for a given compound. The triplet states of both polymers and models have been characterizedin solution by laser flash photolysis, and intersystemcrossing quantum yields have been determined. The triplet yields also decrease with increasing D-A character of the chromophore and with increasing solvent polarity. A comparison of fluorescence and phosphorescence spectra reveals a very small singlet-triplet energy gap. It seems likely that the lowering, by increased D-A character, of the yields of the triplet states that are intermediates in the photooxidative degradation enhances the photostability of aromatic-basedpolyimides. Introduction Aromatic polyimides are important high-performance materials because of their superior thermal stability, excellent mechanical properties, and low dielectric constants. There are many reports of the thermal behavior of high-temperature, heat-resistant polyimides but relatively little is known about their photochemical behavior. We have recently begun a comprehensive study'-6 of the photochemistry and photophysics of a series of polyimides exemplified by 6F-ODA and 6F-6F (see structures and appropriate designations in Chart l),derived by reaction of a bis-aromatic dianhydride, 6F, in which two phthalic anhydride moieties are linked by a hexafluoroisopropylidene "hinge" group, with bridged (either oxygen of hexafluoroisopropylidene)aromatic diamines and by PMDA-ODA, derived from pyromellitic dianhydride and 4,4'oxydianiline. In the course of this investigation, we also conducted a comprehensive study of a series of N arylphthalimides (see structures in Chart 1 designated PA-A, PA-POA, PA-ClA, PA-MOA, and PA-CNA) with differing substituents at the para-position of the N-aryl ring.*?5 These phthalimides model (vide infra) many of the photophysical and photochemical properties of the much less tractable polymers. Broad-band mercury lamp irradiation of polymers such as 6F-ODA, 6F-6F, and 6F-6H in the presence of oxygen results in their complete degradation to small-molecule fragments. For example, 6F-ODA, when irradiated in the presence of oxygen, yields4photoproducts such as benzoic acid, H20, CO, COz, and CF3H: Irradiation of films of 6F-ODA and 6F-6F results in progressive weight loss by clean ablation of the surface. In contrast, irradiation of t Department of Polymer Science.
Department of Chemistry and Biochemistry. Polymer Research Group. e Abstract published in Advance ACS Abstracts, November 15, 1993. 5
0024-9297/94/2227-0832$04.50/0
6F-ODA through a Pyrex filter results in chain breaks but no facile breakdown to small-molecule fragments or surface ablation. Upon examination of model systems, we found that irradiation of model N-arylphthalimides through Pyrex in the presence of oxygen affords phthalic anhydride as the major photopr~duct.~ Phthalic anhydride undergoes secondary photolysis to unknown photoproducts only under broad-band mercury lamp irradiation, which includes light of wavelength less than 300 nm. These observations have led us to propose4 a mechanism for the extensive photooxidative degradation of fluorinated polyimides involving initial production of aryl anhydride functional groups in the irradiated polymer followed by their subsequent photolysis. We have obtained evidence for the first part of this mechanism by direct observation5of aryl anhydride functional groups (by their characteristic IR bands) in 6F-6F films irradiated in the presence of air. It is clear from our previous work on photooxidative degradation of both polymers and models that the least photolabile materials are those in which the donoracceptor (D-A) character of the chromophore is predicted to be the most pronounced. The order of stability of the polymers (as determined by weight loss) is PMDA-ODA > 6F-ODA > 6F-6F and, for the models (as determined by degradation quantum yields) is PA-POA > PA-A = PA-ClA > PA-CNA. Presumably, the pyromellitimide moiety in PMDA-ODA is a more powerful electron acceptor than the 6F moiety in 6F-ODA, and the 6F bridging group in the diamine moiety of 6F-6F suppresses the D-A character of 6F-6F relative to 6F-ODA. The photostability trend for the N-aryiphthalimide models parallels the expected electron-donating ability of theparasubstituent, Le., OPh > H > CN. We have also obtained evidence from quenching experiments that the triplet state is a key intermediate in PA-A photodegradation. In this paper we will report details of the D-A effect on the singlet states of 6F-based polymers and appropriate model 0 1994 American Chemical Society
Photodegradation of Polyimides. 6 833
Macromolecules, Vol. 27, No. 3, 1994 Chart 1
15
r
PMDA-ODA
6F-ODA Wavelength (nm)
Figure 1. Absorption spectra of thin polyimide films cured at 300 O C : (a) 6F-6F (b) 6F-6H; (c) GF-ODA. Table 1. Fluorescence Maxima (nm) of the Polymers in Fluid Solution at Room Temperature polymer methylene chloride tetrahydrofuran acetonitrile 600 not soluble 6F-ODA 595 6FdH 575 580 very weak 6FdF 525 530 542
6FbH
system using loo-, 500-,lo‘-,and l@-A columns. All molecular weights are reported relative to polystyrene standards. 6F4F
0
Results and Discussion 0
6 R=CN, Ran, R=C1, R = OPh,
PA-CNA PA-A PA-CIA PA-POA RsOMe, PA-MOA
R Me, PA-Me R-Pr, PA-Pr
phthalimides, direct observation of the triplet states of these materials in fluid solution, measurements of intersystem crossing quantum yields (@bJ and, finally, lowtemperature emission experiments that enable estimation of singlet-triplet energy differences. Our results suggest that D-A stabilization of the lowest singlet state of both polymers and models reduces photochemical reactivity, to a certain extent, by reduction of intersystem crossing yields.
Experimental Section The synthesis and purification of both polymers and model compounds have been described e1sewhere.l~~ All solvents were obtained from Burdick and Jackson and used as received. UVvis absorption spectra were recorded on a Perkin-Elmer Lambda 6 spectrophotometer and fluorescence spectra on a Spes Fluorolog-2 spectrofluorometer. A Spesphosphorescence attachment was used to obtain phosphorescence spectra at different delay times after pulsed excitation. Fluorescence quantum yields were obtained relative to anthracene ( i p = ~ 0.3) in cyclohexane? Laser flash photolysis experiments were carried out using a Lumonics HyperEx 440 excimer laser as an excitation source operating a t 248 (KrF) or 351 nm (XeF), and an Applied Photophysics (AP) pulsed xenon lamp as monitoring source, the output from which was analyzed using an AP monochromator/fast PMT system as reported earlier.6 Extinction coefficients of the triplet states were determined by the energy transfer method’ from benzophenone. Intersystem crossing yields, ah, were determined relative to benzophenone = 1.0) by laser flash photolysis a t 248 nm. Polymer molecular weights were determined from gel permeation chromatography with a T H F mobile phase on a Waters
A. Fluorinated Polyimides: Photolysis Dependence on Amine Donor Characteristics. The absorption and fluorescence spectra of the polyimides designated 6F-6F, GF-ODA, and 6F-6H are similarto those previously reportedsfor related materials. UV-vis absorption spectra of thin films of these polymers (uniform film thickness) cured a t 300 “C are shown in Figure 1,with the 6F-ODA film being considerably red shifted compared to 6F-6F and 6F-6H. The red shift in the absorption spectra occurs in accordancewith expected increasing D-A interactions, i.e., 6F-6F < 6F-6H < 6F-ODA. Solutions and thin films of the 6F-based polymers exhibit a broad, weak fluorescence maximum. For example, 6F-6F has a maximum a t 542 nm in acetonitrile, 530 nm in tetrahydrofuran, and 525 nm in CH2C12. The results are typical for the 6Fbased polymers, all of which exhibit a red shift in the fluorescence peak maximum with increasing solvent polarity. Of particular interest in the present case is to compare the shift in the fluorescence peak maximum in a common solvent for 6F-6F, 6F-6H, and 6F-ODA. The entries in Table 1show fluorescence emission maxima for all three polymers in at least one solvent: A shift in the peak maximum to lower energies with the expected order of increasing D-A interaction (6F-6F < 6F-6H < 6FODA) apparently occurs in each solvent. In addition, the intensity of the triplet spectrum of all three polymers in CH2Cl2 suggests that the intersystem crossing yield to the triplet state of the aryl imide is greatest for 6F-6F and least for 6F-ODA, with 6F-6H being intermediate. This assumes that the extinction coefficientsof the triplet states are approximately equal. On the basis of the donor character expected for the amine-based component of the aryl imide repeat unit (ODA > 6H > 6F), we might expect to see the photolytic decomposition,which we will demonstrate in a subsequent section in this paper to proceed, a t least to an extent, from the triplet state, to follow the same order; i.e., 6F-6F would be the more photolabile polymer. In accordance with this prediction, photolysis of 6F-6F films led to very rapid decomposition compared to 6F-6H and 6F-ODA films. The rapid decomposition
834 Creed et al.
Macromolecules, Vol. 27, No. 3, 1994
L 80000 7 I
Scheme 1. Mechanism for the Photolysis of N-Arylphthalimides
1
1.h 2.1%
* C
5
,-
WO-SLYSIS
,
15
2c
T V E (h)
Figure 2. Plot of molecular weight of 6F-ODA (O),6F-6F (a), and 6F-6H (*) films at peak maxima (compared to polystyrene) versus photolysis time with an unfiltered medium-pressure Hg lamp in air. of 6F-6F is even more surprising when we consider that its UV-vis absorption (not shown) in the range of the photolysis source (unfiltered medium-pressure Hg lamp) is much less than for 6F-6H and 6F-ODA. Figure 2 shows changes in the molecular weight at the peak maximum for 6F-ODA, 6F-6F, and 6F-6H as a function of photolysis time in air. As indicated above, the order of decomposition is 6F-6F > 6F-6H > 6F-ODA. Obviously, the photostability of the films parallels the D-A acceptor properties of the photoreactive repeat unit and the apparent intersystem crossing yield to the triplet. However, we should point out that a rough analysis of the relative intersystem crossing yield for 6F-6F, 6F-6H, and 6F-ODA does not exactly parallel in magnitude the large difference in decomposition rate exhibited in Figure 2 for 6F-6F versus 6F-ODA and 6F-6H. While quantitative results are not available, our results seem to indicate that the substituent effect on the N-aryl ring ((CF&CPh versus (CH&CPh or OPh) not only alters the intersystem crossing yield but also has an effect on the subsequent reactions which proceed from the triplet state. Hence, any proposed photodegradation mechanism, such as depicted in Scheme 1 (suggested5 as being consistent with our cumulative results to date for model and polymer systems having the N-aryl imide moiety), must reflect the possibility that the R substituent group can affect both the properties of the excited state from whence products are formed and subsequent reactions of any intermediates, such as a biradical comprised of an amidyl and a carbonyl radical, which may form. This will, of necessity, require positive identification of the spin multiplicity of the excited state leading to product formation. The rest of this paper will be directed toward identification of the effect of D-A properties on the photophysics and photochemistry of model N-arylphthalimides by clearly defining the spin multiplicity of the reactive states, the D-A effects on singlet emission and singlet to triplet intersystem crossing yields of N-arylphthalimides, the energy levels of the singlet/ triplet states of N-arylphthalimides, and the lifetimes of the reactive states. B. Photophysics of Model N-Arylphthalimides. Absorption spectra of the N-arylphthalimides (see structures) shift to longer wavelengths as the D-A character increase^.^ Furthermore, there is a measurable solvent effect on the fluorescence (first five entries in Table 2) of each of these arylphthalimide models: The emission maxima shift to lower energies with increasing solvent polarity. We note that the N-aryl group seems to be essential for observation of the broad, substituent and solvent sensitive band in the fluorescence spectra of the
Table 2. Fluorescence Maxima (nm) of Phthalimide Model Compounds in Fluid Solution at Room Temperature compd
cyclohexane
methylene chloride
PA-CNA PA-A PA-ClA PA-POA PA-MOA PA-Pr PA-Me
471 507 509 555 565 410 406
493 537 540 606 622 410 410
acetonitrile 512 552 555 610 a 410 410
Weak emission between 610 and 650 nm.
N-arylphthalimides, since the fluorescence of N-alkylphthalimides (last two entries in Table 2) is considerably blue shifted relative to the N-arylphthalimides and shows little solvent sensitivity. For example, the fluorescence maximum of PA-Pr in cyclohexane is at 410 nm while it is at 507 nm for PA-A. Furthermore, in contrast to PAA, the fluorescence maximum of PA-Pr does not shift when the solvent is changed from cyclohexane to acetonitrile, although its intensity increases considerably. The fluorescence maximum of PA-A shifts from 507 to 552 nm in going from cyclohexane to acetonitrile, with a significant drop in intensity (Table 2). To provide additional insight into the photophysical nature of N-arylphthalimides, we measured the fluorescence quantum yields (+F) of the models in several different solvents and at excitation wavelengths from 250 to 350 nm (Table 3):We did not observe an excitation wavelength effect on +F for any of the models or polymers examined. Moreover, the fluorescence was weak in all cases, with the highest value of CPF being 8.3 X l v f o r the model compound PA-CNA in cyclohexane. Upon close examination of the
Macromolecules, Vol. 27, No. 3, 1994
Photodegradation of Polyimides. 6 835
Table 3. Quantum Yields. of Fluorescence of Phthalimide Model Compounds in Fluid Solution at Room Temperature compd
PA-CNA PA-A PA-ClA PA-POA
PA-MOA PA-Pr PA-Me a
cyclohexane 8.3 X 103 2.5 X lo3 2.3 X lo3 3.5 X lo-' 3.5 X lo-' 5.2 X 10-5 4.3 x 105
methylene chloride 4.6 X 5.6 X 4.9 x 4.9 x 3.9 x 1.1x 9.4 x
103 lo-' 10-4
10-5 10-5
10-3 10-4
1
In.
acetonitrile 1.8 X 10-3 2.5 X lo-' 2.5 X lo-' 2.8 X 10-5 1.8 x 10-5 1.0 x 10-3 1.4 x 10-3
Relative to anthracene (@JF = 0.3); & l o % .
results in Table 3, it can be seen that CPF diminishes for a given compound in more polar solvents and as the D-A character of the material is enhanced by electron-donating substituents. In contrast to the results for the Narylphthalimides, the "isolated" phthalimide chromophore, as exemplified by PA-Pr, exhibits a marked increase in CPF from 5.2 X to 1.0 X in going from cyclohexane to acetonitrile. Solvent effects on the fluorescence of N-alkylphthalimide~~ have been attributed to an increase in a,?r* character of the singlet state in solvents of increasing polarity. This seems quite reasonable to us. Obviously, the same mechanism cannot be invoked for theN-arylphthalimides or for the 6F-6F, 6F-6H, and 6FODA polymers since an increase in solvent polarity leads to a pronounced red shift in the position of the wavelength maximum and an accompanying decrease in the fluorescence intensity. This is best exemplified for 6F-6F in Table 1since we were able to obtain fluorescence spectra in three solvents for this polymer. From the results presented in this report, it seems obvious that D-A effects are important to the singletstate behavior of N-arylphthalimides and aromatic-based polyimides. One possibility is that N-aryl imide groups have a twisted intramolecular charge transfer (TICT)state in which the N-aryl group acts as an electron donor and the phthalimide as an acceptor. Indeed, Frank et al.12-14 have previously suggested that the red-shifted fluorescence of N-aryl polyimides originates from intramolecular charge transfer complexes. While there is no direct proof that N-arylphthalimides exhibit a TICT state, they do have a very broad, weak, structureless fluorescence that is dependent on the donor-acceptor character of the interactive components and on solvent polarity, with a large Stokes shift from the onset of absorption, all characteristic of a TICT state.lOJ1 A compound forming a TICT state is also expected to have a very small singlet-triplet energy gap: Such is apparently the case for N-arylphthalimides. For example, the phosphorescence spectrum of PA-CNA at 77 K (Figure 3) in an ether-tetrahydrofuran glass, shows a 0-0 band at 402 nm, corresponding to a triplet energy, ET,of 71.1 kcal-mol-l. Such precise determination of the singlet energy, Es, is not possible, since no vibrational structure is apparent in the fluorescence spectrum. However, taking the wavelength at which the fluorescence intensity is 10% of its value at ,A, as an approximation for the position of a 0-0 band, we obtain a value for Es of ca. 72 kcal-mol-l in cyclohexane and ca. 70 kcal-mol-l in acetonitrile, identical within experimental error to the triplet-state energy obtained from the phosphorescence experiments (Table 4). Similar results are obtained for the other model compounds and for 6F-6F and 6F-ODA (Table 4). C. Spin Multiplicity of Reactive State. Although we have presented data for N-arylphthalimides with respect to the effect of donorlacceptor substituentson the singlet state, we are ultimately interested in the triplet state, since preliminary quenching results suggest5 that
500 Wavelength (nm)
400
700
600
Figure 3. Phosphorescence spectrum (Aex = 250 nm) of PACNA in ether-tetrahydrofuran glass at 77 K. Table 4. Excited-State Energies from Emission Spectra of N-Arylehthalimides and Polymers
Es,kcabmol-1 compd
PA-CNA PA-A PA-POA 6F-6F 6F-ODA
ET,^ kcalemol-1
cyclohexane
acetonitrile
71.1b 67.6' 67.1' 71.5b 71.1b
72.2c 69& 64.OC
69.6c 65.4' 58.7c
film
tetrahydrofuran
67.gC 72.2c 65.gC d
Ether-tetrahydrofuran glass at 77 K. Estimated from 0-0band. Estimated from onset of emission (wavelength at which intensity of corrected spectrum is 10% of maximum intensity). d Not determined.
photooxidative degradation of the phthalimide model compounds proceeds from the triplet state of the imi,de. Therefore, we have used laser flash photolysis to characterize the lowest triplet states of the polymers and model compounds in fluid solution. Laser flash photolysis (248 nm) of the models PA-CNA, PA-A, PA-ClA, PA-POA, and PA-MOA led to observation of transients with lifetimes of several microseconds in nitrogen-saturated solvents. The transient lifetimes are greatly reduced in each case when the medium is air saturated. For example, the transient from PA-A in cyclohexane has a sharp maximum at ca. 300 nm and a broad maximum at ca. 520 nm (Figure 4). The lifetime of this transient is ca. 12 p s in a nitrogen-purged solution and ca. 150ns in the presence of air. The decay of the transient absorption is independent, within experimentalerror (&lo%),of the monitoring wavelength, indicating that it is due to a single species. The transient is readily quenched by cyclohexadiene (ET = 52.4 kcabmol-l) with a quenching rate constant of 6.0 X lo9L-mol-l-s-land by naphthalene (ET= 61 kcal-mol-1) with a quenching rate constant of 2.9 X lo9 L.mol-l4. Quenching with biphenyl (ET= 66 kcal-mol-') is much less efficient (
